sometimes I get "charged" and the next thing I touch something that conducts electricity such as a person, a car, a motal door, etc I get shocked by static electricity.

I'm trying to avoid this so if I suspect being "charged" I try to touch something that does not conduct electricity (such as a wooden table) as soon as possible, in the belief that this will "uncharge me".

Is it true that touching wood will uncharge you?

How and when do I get charged? I noticed that it happens only in parts of the years, and after I get out of the car...

9 Answers
9

My brother, an electrical engineer, used to carry around a 1 megaohm resistor
during the dry winter months when you easily get a shock after walking across a carpet and touching a light switch or another person. If you hold one lead of the resistor in your hand and touch the light switch or whatever else you are touching with the other the discharge turns from a nasty shock into a very mild and kind of amusing fizzle. It's kind of fun and I'm sure you can find a resistor with large enough resistance lying around in your local physics lab, so give it a try! Of course I don't know anybody geeky enough to actually use this as a practical solution.

I've done something similar, adding a few 'grounding points' I can touch before e.g. using my computer (which has a metal USB peripheral that will crash if I touch it while carrying a static charge). 1 Megaohm is not enough to prevent the shock, I can still draw sparks on these grounding points. I'm going to try resistors with higher values.
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HobbesJan 23 at 18:24

Free or nearly-free electrons on stuff like wool are getting rubbed off onto you so that your body holds some sort of net total electric charge. When you touch a metal door (or any piece of metal) then the electrons want to spread out to balance themselves between you and the metal. Since the metal conducts electricity very well, they fly off you very quickly which heats up the air in between you and the metal, giving you the painful "shock" feeling.

Wood won't uncharge you very well, since it doesn't conduct electricity very well. The only ways to prevent getting shocked are either not building up charge in the first place, or constantly touching metal so that the charges get released way before they can build up - in effect spreading out your shocks to many smaller shocks you can't feel.

There are many ways to build up a static charge, but it is generally much, much easier when the air is dry. Since cold air is drier, this means you probably build up a charge more quickly in the winter.

As for how you build up charge in the first place, it is usually by rubbing certain materials together. Plastic or rubber rubbing against wool carpeting or clothes (or any sort of hair) will do it, which is the most common cause for people in their day-to-day lives. If you want to stop it, you could consider using dryer sheets, which use a substance which happens to be conductive to soften your clothes - this makes you constantly discharge as you touch your own clothes, so you achieve the "many tiny shocks" method alluded to above. Or, you could use a humidifier in your home, which adds water to the air, making static much harder to build up.

That's incredibly annoying, I agree. Can I make a joke with physics content here? I always thought a great idea would be to carry a capacitor with you. Then, from time to time, you discharge yourself with it (probably with a resistor, so that you actually avoid getting shocked), charging the capacitor. When you pass by that person you don't like very much, you discharge the capacitor at him. See?, two problems solved at once.

:)

Now that the joke is told, let me try to answer the questions:

Is it true that touching wood will uncharge you?

No, you have to touch a conductor. Wood won't work.

How and when do I get charged? I noticed that it happens only in parts of the years, and after I get out of the car...

It happens when the air is very dry. I used to live in a city where the humidity in the air is huge and that never happened. In the winter the air gets very dry and these shocks are more common. So, you can move to a rain forest and your problem will also be solved.

Comment from @Donna who does not have enough points to comment: "I live on the west coast of Canada, which is a temperate rainforest and has very high (90%) humidity. I still get shocked all winter. In an office of 4 people, I'm the only one it happens to - and it has happened to me my entire life. I hate it, I'm afraid to touch the filing cabinet. Even the tiny shocks are shocking and annoying."
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anna vDec 3 '12 at 20:36

1

@annav: A lot depends on the temperature difference between inside and out. It may be quite humid outside, but if the inside temperature is much higher, as it is in winter, the air inside will have low humidity, so it will let your charge build up.
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Mike DunlaveyDec 3 '12 at 21:21

I have to be careful when greeting my wife, to touch something like the screw on a switchplate first :)
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Mike DunlaveyDec 3 '12 at 21:23

Every time two dissimilar insulators rub together, there will be a relative buildup of charge due to the triboelectric effect. In essence the two materials stick together briefly, and when they unstick the electrons may prefer to stay with one surface and not the other. This happens summer or winter.

The reason you get "shocked" more in winter has to do with the conductivity of the air - which in turn is largely determined by the absolute moisture content. Not the relative humidity, but the absolute. You can see some evidence of this in an earlier answer on a related topic. This is why the effect is usually worse in winter - when the external temperature is lower, the saturated vapor pressure is lower as well. So even when it's raining outside, if it's only 1°C the absolute humidity will be low.

As for why this happens more to some people than others: mostly this is due to clothing preferences - some materials are just more susceptible to the effect (more electrically dissimilar than others). The fact you often notice this when getting out of a car in winter can be explained in part by (a) materials of the car seat, (b) you "slide" off the car seat, so lots of friction, and (c) you touch metal immediately after charging up, so there is less time for the charge to dissipate.

As for "how to prevent": in environments with sensitive electrical components the floor is usually treated with a (slightly) conductive compound, and people wear a heel strap that connects their skin (around the outside of the shoe) to the floor - see this image from ultrastatinc.com:

They also wear a conductive wrist strap that is grounded at the workstation (usually with a high resistance in series, so if they touch a live part they don't get electrocuted).

But the rest of us mortals have a few other options. Humidifiers (keep the air moist - also helps your skin) are very effective. The choice of shoes, clothing, and flooring material can help a bit (treat your carpets with antistatic spray). But when you are out and about, and you fear touching that metal door handle, I usually use one of these tricks:

hit the object quickly with the back of your hand (where you have fewer nerve fibers). You still get the discharge, but your body can't tell the difference between the shock and the slap (you do have to hit fairly hard for this to work).

Touch the object with a poor conductor - the current will flow more slowly. Product idea for engineering colleges: make your class ring "stone" out of a slightly conductive material so it can serve this purpose - geeky and practical. Touch the door with your ring before you go through

If you have a key, coin etc, you can touch the metal part with that: the "spark" (which is more painful) will jump to the coin, and then the current will distribute over a wider area (the fingers holding the coin). Quite effective at reducing the local pain of the shock, although you still get a little jolt up your hand. The key is my preferred method - easier to find in my pocket, and usually needed anyway when I approach a door.

Whether touching wood will help you is questionable. Usually dry wood is not a good conductor - but depending on the surface finish, wood may be somewhat conductive. From brittanica.com:

Oven-dry wood is electrically insulating. As moisture content increases, however, electric conductivity increases such that the behaviour of saturated wood (wood with maximum moisture content) approaches that of water.

Thus wood may be a suitable high resistance medium for a slow discharge - but unless you specifically made a piece of wood of known properties, I would not count on it.

This happens to me all the time; I collect an electrostatic charge just sitting at the computer; earlier Sir Dumpty often was the medium for discharge - an audible zzzt as my accumulated charge discharged through his snout when he came to snuffle against my arm/hand/palm.

A simple solution would probably be to touch the floor/earth; do not touch metal if you can avoid it.